As scaling down of devices progresses, nonvolatile semiconductor memory devices are becoming smaller in size. Among nonvolatile semiconductor memory devices, there are floating-gate type devices and MONOS (Metal Oxide Nitride Oxide Silicon) type devices each having a charge storage film for storing charges into memory cells.
As the devices are becoming smaller, the charge storage film is expected to become thinner while maintaining high charge storage properties, so as to facilitate manufacturing and restrain interactions between adjacent cells.